Conventional loudspeakers produce audible sounds by displacing air via the movement of a diaphragm. Specifically, the diaphragm is attached to and moves under the control of a voice coil, through which electric currents associated with the sounds to be reproduced are driven. The voice coil is disposed in an annular air gap of a magnetic structure. This structure includes a permanent magnet that provides radial flux in the air gap. Current through the coil interacts with this flux to provide axial forces on the coil and thereby displace the coil and attached diaphragm in accordance with the instantaneous magnitude and direction of the current.
For a number of years, the air gaps in some loudspeakers have been filled with magnetic fluids, sometimes referred to as "ferrofluids." These ferrofluids, which contain suspended magnetic particles, transfer heat from the voice coil and also provide damping for the movement of the coil, thereby reducing distortion in the speaker and smoothing its frequency response. For further explanation, see W. Bottenberg, L. Melillo and K. Raj, "The Dependence of Loudspeaker Design Parameters on the Properties of Magnetic Fluids," Journal of Audio Engineering Society, Volume 28, January/February 1980, pp. 17-25. The ferrofluids are particularly useful in speakers that move their voice coils relatively large distances, such as woofers and sub-woofers which respond to low frequencies. See, for example, L. Melillo and K. Raj, "Ferrofluids as a Means of Controlling Woofer Design Parameters," Journal of Audio Engineering Society, Volume 29, No. 3, 1981 March pp. 132-139.
To obtain the advantages discussed in the two articles cited above, the ferrofluid should remain in the voice coil air gap, rather than flowing out of the gap during voice coil excursions. The ferrofluid is attracted by the magnetic field in the gap and is drawn into the gap to form a ring between the voice coil and the respective walls of the magnetic structure. The ferrofluid is held in place by the magnetic field, and thus, resists flowing out of the gap. During large voice coil excursions, ferrofluid adhering to the voice coil is carried out of the gap and beyond the magnetic field region. The ferrofluid may be removed from the voice coil by acceleration forces, and thus, splash on nearby components.
The gap is connected to a cavity between the speaker magnet and various components of the magnetic structure and because the ferrofluid acts like an O-ring seal between the voice coil and the magnetic structure the cavity is essentially sealed. As the voice coil moves, it tends to elevate the air pressure within this cavity. If this pressure builds up to a point where it exceeds the pressure capacity of the ferrofluid O-ring seal, the air bursts through the seal and relatively large amounts of the ferrofluid may then be blown or flow out of the gap.
The build up of air pressure in the cavity is affected by the pumping action of the voice coil and the air temperature within the cavity. This pressure build up is alleviated somewhat by venting the cavity, if the vents are large enough. Vents tend to handle the pressure well for speakers in which the movements of the voice coil are relatively small, such as speakers with low excursion, low- to mid-frequency drivers. However, the vents do not as readily handle the pressure associated with the larger voice coil movements in large size woofer or sub-woofer speakers.